unfortunately this is a Bug right now. Please reinstall your ESXi host with 5.5.0 without Update 1. The fault is related to iSCSI software driver in Rollup image.
Please check link below for futher information:
Grzegorz aka CZAMP
Thank you for the information. This helped a LOT!!
I have set my system up to address the QNAP TS-870 via NFS until this is resolved.
I still have my ESXi server configured to use NFS with my QNAP TS-870.
I have not worked anymore to get iSCSI going - though I would like to at some point.